This invention, in general, relates to methods and apparatus for processing sampled data signals; and more particularly, it relates to ground mapping methods and apparatus which are used in synthetic aperture radar.
Conventionally, to form a ground map via synthetic aperture radar, an airplane flies over the ground that is to be mapped and transmits a sequence of radar pulses at the ground. Return radar signals from the transmitted pulses are sampled in the airplane; those samples are then compressed in the airplane via various data compression techniques; and the compressed data is then transmitted from the airplane to a receiving station on the ground. Alternatively, the original uncompressed samples are sent to the ground station. Data compression is performed to reduce the amount of information that needs to be transmitted, and thereby reduce the bandwidth of the transmission channel.
If the compressed data is received in the ground station, it is then decompressed in order to reconstruct, to close approximation, the original data samples. Then, the reconstructed data samples, or the original samples if sent in uncompressed form, are further processed by fast Fourier transform methods (FFT methods) and inverse FFT methods in order to produce the ground map. Alternatively, the reconstructed data samples or original data samples are convolved with a sampled data function in order to produce the ground map.
A problem, however, with these prior art mapping procedures is that they are very complicated, and thus very slow to carry out. Particularly time-consuming are the steps of processing the reconstructed or original data samples via an FFT, inverse FFT, or convolution operation. Such operations, when performed by a computer, require scores of floating point multiplications and floating point additions.
Accordingly, a primary object of this invention is to provide an improved method and apparatus for forming visual images via radar, such as ground mapping, by which the complexity and time for forming such images is greatly reduced.
Also, another more general object of the invention is to provide a fast yet simple method and apparatus for processing a sampled data input signal of any source, radar or nonradar.